Actually, in theory, ANY modification you do to your vehicle is 'illegal' because the type approval no longer technically applies - EEC Eurocrats wanted it so you couldn't even change the stereo from standard...

Will have to watch out for the chipped Focus on my way home then If he thinks I'm gonna sit there revving the tits of my car stationary, then he's got another thing coming

Worth considering though, that if you didn't have 'fancy' plates on your car, you probably wouldn't have got pulled......